A study of fractographic features of fracture surfaces, patterns of structural-phase transformations, and mechanisms of plastic deformation and fracture of low-activation vanadium alloy V–Cr–W–ZrC in the process of toughness tests was carried out. The appearance of a qualitatively new (non-dislocation) mechanism of plastic deformation was revealed—the mechanism of bcc → hcp → bcc transformation with a change in the systems of reverse transformations and (or) the participation of quasi-viscous mass transfer in the fields of high local pressure gradients. An important feature of this mechanism is its activation at the nanoscale level with the formation of nanovolumes several nanometers in size—new carriers of homogeneous transformation deformation of the Bain type. A significant feature of these carriers is the absence of any effective obstacles such as dislocations or disorientation boundaries for both homogeneous tensile/compressive deformation and quasi-viscous mass transfer. The activation of bcc → hcp → bcc transformations as a new non-dislocation deformation mode is based on the phenomenon of phase instability of the bcc crystal in fields of high local stresses and high local gradients of the nanoscale level. The above transformations (both direct and reverse) can be carried out under conditions of thermodynamic gain with a local (in the transformation zone) decrease in energy in the transformation region. This leads to intense softening of the material and high deformation and relaxation rates of highly defective substructures of deformation and deforming and local internal stresses.

The features of microstructure and mechanical properties of yttrium-modified low-activation austenitic steel after high-temperature aging at 700 °C during 100 h are investigated. Using transmission and scanning electron microscopy, it is shown that aging results in the precipitation of dispersed particles of M23C6 carbides (M–Cr, Mn, Fe). These particles are found along the grain boundaries and inside the grains in the form of shells on dispersed particles of MC carbides (M–Ti, Ta, V), as well as at the boundaries of micro- and nanotwins. The strength properties of the steel are studied in tensile tests at 20, 650 and 700 °C. It is shown that an intense precipitation of carbides has a significant effect on the mechanical properties of this steel. After aging of the quenched state, the yield strength increases at all test temperatures used in this study, while the elongation to failure decreases by a factor of 1.6. This is due to the precipitation of carbide particles. After aging of the cold-rolled state, the yield strength decreases by 1.3–2 times, while the values of elongation to failure do not practically change. This is due to the intensive recovery of the dislocation substructure and the prevailing precipitation of carbides along the boundaries of micro- and nanotwins under aging of the cold-deformed state.

The effect of aging at 700 degrees C for 100 h on the microstructure, phase transformations, and mechanical properties of a new high-manganese low-activation austenitic steel is studied. It is shown that a long-term high-temperature exposure results in the precipitation of dispersed particles of M23C6 carbides. The formation of these particles occurs at grain boundaries, incoherent and coherent twin boundaries, and inside grains. Particles precipitated along grain boundaries have the form of discontinuous or continuous thin films. At incoherent twin boundaries, M23C6 carbides precipitate in the form of parallel thin plates lying along the {111} crystallographic planes of austenite at a certain angle to these boundaries. At coherent twin boundaries, these particles have the form of thin plates parallel to the twinning plane, and their transverse dimensions are limited by the thickness of twins. Inside grains, M23C6 particles can have the shape of rhomboids or cuboids. The grain and twin structure remains stable during aging. The precipitation of dispersed carbides affects the strength and plastic properties of the steel. After aging of the quenched steel, the yield strength increases slightly, while the elongation decreases by 1.5-1.8 times. After aging of the cold rolled steel, the yield strength decreases by 1.3 times at all studied temperatures (20, 650, and 700 degrees C). In this case, the elongation to failure at 20 degrees C decreases by 1.7 times, while at high temperatures its values change little. The effect of dispersed M23C6 particles on the strength and plastic properties of the steel is discussed.

The microstructure of 12% chromium ferritic-martensitic steel EK-181 formed during thermomechanical treatment with deformation at 900 degrees C is investigated. It is shown that this treatment leads to a refinement of the parameters of the steel microstructure and an increase in the dislocation density and the magnitude of microdistortions of the crystal lattice, compared to the state after quenching. Plastic deformation at 900 degrees C followed by tempering allows reducing the average sizes of the prior-austenite grains by 2 times, the width of martensite laths by almost 4 times, and the particles of the carbide phase M23C6 by 2 times, compared to the state after traditional heat treatment. In this case, the density of dislocations and the degree of lattice microdistortions increase by 2 times. The microhardness of EK-181 steel is also studied. It is shown that plastic deformation at 900 degrees C leads to a 15% increase in the steel microhardness in the quenched state and by 10% after additional tempering.

A study of the influence of annealing in the range from 700 to 1600°С on the microstructure features and microhardness values of the V–Cr–W–Zr alloy after thermomechanical treatment was carried out. It was found that, as a result of rolling in the V–Cr–W–Zr alloy, texture fibers α, γ, θ are formed. It has been established that the structural state after thermomechanical treatment is stable up to 800°С. At 900°С, primary recrystallization is activated, which at 1000°С covers the entire volume of the material. Collective recrystallization processes occur in the temperature range of 1100–1400°С. Secondary recrystallization is activated at 1500°С. Under conditions of primary, collective, and secondary recrystallization, against the background of orientational grain growth, the disappearance of the texture components of the θ-fiber is observed. It was established that, in the temperature range of 1500–1600°С, a partial redistribution of W occurs. The influence of the average grain size on the microhardness values of the alloy was analyzed. The main mechanisms of material strengthening are discussed.

The peculiarities of phase transformations in dispersion-strengthened low-activation chromium-manganese austenitic steel under high-temperature heating and cooling are studied using in situ X‑ray diffraction (XRD) and transmission electron microscopy (TEM). The presence of ε‑martensite plates in this steel after ageing (700 °C, 100 h) is shown. This phase is formed under local internal stresses arising from intensive growth of dispersed particles of M23C6 carbides (M – Cr, Mn, Fe) during the steel cooling from the aging to room temperature. The comparatively low stacking fault energy (25 mJ/m2) of steel and the low stresses of ε‑martensite formation favor the γ→ε transformation. The in situ studies show that under subsequent high-temperature heating up to 500–590 °C, the ε‑phase completely transforms into austenite. During cooling from 590 °C to room temperature, this phase appears again. The formation of ε‑martensite does not affect the structural-phase stability of austenite near the temperature range of anticipated operating conditions.

TEM studies were performed to examine the effect of holding of dispersion-strengthened heat-resistant reduced activation 12

The effect of annealing at 700 degrees C for 50 h of solution-treated (ST) and cold-rolled (CR) samples of low-activation high manganese austenitic steel on its phase composition, microstructure and microhardness is investigated. Carbide subsystem of the steel undergoes significant changes following annealing. After ST, it causes precipitation of M23C6 particles along grain boundaries and on MC particles. Annealing of CR samples leads to the formation of these particles at microtwin boundaries. There are practically no changes in the size and volume fraction of MC carbides after annealing. A depletion of the steel solid solution in carbon due to M23C6 carbides precipitation decreases the austenite stability towards the gamma-epsilon martensitic transformation. This leads to the formation of epsilon-martensite after CR + annealing. The epsilon-martensite plates are, presumably, formed as a result of relaxation of local internal stresses generated nearby large M23C6 particles during cooling from 700 degrees C. Microhardness of the steel does not practically change after its annealing. This is due to simultaneous processes of recovery and precipitation of dispersed M23C6 carbides. The former process contributes to a decrease in the substructural strengthening efficiency and the latter - to tan increase in the efficiency of dispersion strengthening.

The authors investigated the patterns of fracture during impact bending tests and determined the values of impact strength and temperature of the ductile-brittle transition in temperature range from –196 to 100 °С of heat-resistant 12 % chromium ferritic-martensitic steel EP-823 in structural states after traditional heat (THT) and high-temperature thermomechanical (TMT) treatments. After THT, temperature of the ductile-brittle transition Tdbt is approximately –45 °С, after HTMT – approximately –40 °С. At these temperatures, the impact energy (KCV) after THT is approximately 36 J/cm2, after HTMT – 32 J/cm2. Fractographic studies conducted by scanning electron microscopy of the fracture features of impact steel samples after two treatments (THT and HTMT) in the low-temperature test area (at cryogenic temperatures) showed a predominantly brittle nature of fracture, while fracture occurs by the mechanism of a transcrystalline quasi-cleavage. In the temperature range of the ductile-brittle transition, a mixed nature of fracture is observed, which passes through the mechanism of a transcrystalline quasi-cleavage with elements of ductile dimple fracture. In the temperature range from 50 to 100 °С, the extremely ductile nature of the fracture was detected, realized by the transcrystalline dimple fracture mechanism. After HTMT, there is a slight decrease (relative to THT) in the steel impact strength in almost the entire temperature range under consideration and, accordingly, an increase in the temperature of its ductile-brittle transition. This is due to the tests’ geometry, in which the direction of impact occurs in the plane of the layered structure, and it facilitates the formation of delamination cracks.

The effect of the modes of thermomechanical treatment (TMT), involving warm plastic deformation (e = 2) by rolling in the temperature interval of 600 – 900°C, on the austenitic steel microstructure is studied. Using the methods of transmission electron microscopy, it is shown that this plastic deformation results in the formation of a highly defective fragmented microstructure with a high density of low-angle misorientation boundaries. Numerous grains/subgrains flattened in the rolling plane and elongated in the rolling direction are revealed. It is shown that as the TMT temperature is increased, the fraction of low-angle boundaries decreases and so does the dislocation density. It is noted that the average transverse subgrain size varies from 112 nm (TMT-1) to 306 nm (TMT-4). These peculiarities of the steel microstructure ensure advanced strength properties as a result of all TMT modes. The optimal mechanical properties are achieved in this steel after TMT-1, in which case the resulting microstructure is highly homogeneous with a high dislocation density retained; the yield strength values are 808 MPa at 20°C and 516 MPa at 650°C, with the elongation to failure of about 7

The characteristics of the relaxation and recrystallization processes in pure tantalum deformed by rolling at room temperature are studied. It is established that at 800°C the recovery processes are activated and nuclei of recrystallization grains appear. After 1000°C, an activation of primary recrystallization is observed, which, after annealing at 1200°C, involves the entire material volume. It is shown that the processes of structural relaxation beginning at 800°C are accompanied by a dramatic decrease in microhardness.

The results of investigations of the effect of thermomechanical treatment (TMT) regimes on the regularities of phase transformations, microstructure, and mechanical properties of low-activation alloys of the V–4Ti–4Cr–(C, O, N) system are summarized. The mechanisms of these transformations and the relationship between the microstructure and the level of strength and plasticity are established. The TMT regimes are presented that provide a uniform bulk distribution of nanosized particles of stable oxycarbonitride, a significant increase in their density (dispersion), and an increase in the thermal stability of the microstructure. It is shown that these regimes lead to a significant (by 30–60

The results of experimental and theoretical investigations of the thermal stability of heterophase structure of low-activated vanadium alloys with various types of nonmetallic phases (TiC, ZrC, and ZrO2) in the temperature‒time intervals of their technological treatment and operation in the core of nuclear and thermonuclear reactors are presented. It is shown that the thermal stability of these phases is determined by their thermodynamic stability and coagulation rates, which are responsible for the thermal stability of the nanosized heterophase structure. An important factor controlling these rates is the ratio between the standard thermodynamic potentials of their formation and the corresponding potentials of vanadium carbides and oxides. An assumption is made about the possibility of increasing the thermal stability of the heterophase structure of V–ZrC alloys by 100–200 K compared to V–TiC alloys and by 200–300 K in V–ZrO2 compositions.

The paper investigates the influence of dispersion hardening of the V–Cr–W–ZrC alloy on the temperature dependence of its yield strength at different tendency to non-dislocation homogenous distortion enabling BCC → HCP → BCC transformation by the Bain deformation mechanism. The discussion concerns the homogenous deformation mode and dislocation mechanisms in plastic deformation during the formation of strength properties (heat resistance) and low-temperature plasticity of vanadium alloys.

A new low-activation austenitic steel with a modified composition and high austenite stability is proposed. The features of its microstructure after solution treatment (ST) and cold rolling (CR) are studied. The mechanical properties and features of the fracture behavior of this steel under tensile tests in the temperature range of 20–750 °C are discussed. After ST, an austenitic structure with stacking faults and dispersed carbide particles of the MC and M23C6 types is observed in the steel. After CR, the grains are refined, and the average grain size decreases from 41.4 µm (after ST) to 33.9 µm. High-density microtwin packets form in the material, and the dislocation density increases relative to that after ST. As the test temperature increases from 20 to 750 °C, the yield strength of the steel decreases by approximately two times, from ≈300 to 150 MPa (for ST) and from ≈700 to 370 MPa (for CR). In the studied temperature range, the steel demonstrates up to 2.6 times higher values of elongation to failure, ≈40–80% (for ST) and ≈13–27% (for CR), compared to steels of similar compositions and lower manganese content. Mechanical twinning contributes to the high steel ductility up to 300 °C. Signs of discontinuous flow in the tensile curves after ST in the temperature range of 500–600 °C and a decrease in the elongation to failure in the close temperature range indicate dynamic strain aging (DSA). Steel fracture after tension at all test temperatures mainly occurs via a ductile dimple transcrystalline mechanism with elements of ductile intercrystalline fracture. It is shown that cracks nucleate on clusters of dispersed second-phase particles. The mechanisms of plastic deformation, fracture, and strengthening of the proposed modified low-activation austenitic steel are discussed.

A comparative study of the features of the structure and characteristics of the mechanical properties of the V–Cr–W–Zr alloy depending on the modes of thermomechanical treatment has been carried out. It has been established that, compared with the standard mode of thermomechanical treatment, the use of the modified mode leads to a more than twofold decrease in the average grain size. The influence of the temperature of homogenizing annealing on the grain sizes after thermomechanical treatment has been revealed. Such a transformation of the structural-phase state when using a modified mode of thermomechanical treatment contributes to an increase in the short-term strength by ≈12 and ≈21

Reduced-activation ferritic-martensitic steels (RAFMS) are considered not only as structural but also, under certain conditions, as plasma-facing materials for fusion installations. The base material of RAFMS is iron. These steels also contain 8–12 wt

The behavior of the EK-181 low-activation ferritic-martensitic reactor steel (Fe–12Cr–2W–V–Ta–B) in the states with different levels of strength and plastic properties after traditional heat treatment (THT) and after high-temperature thermomechanical treatment (HTMT) in the temperature range from −196 to 25 °C, including the range of its cold brittleness (ductile–brittle transition temperature, DBTT) is studied. The investigations are carried out using non-destructive acoustic methods (internal friction, elasticity) and transmission and scanning electron microscopy methods. It is found that the curves of temperature dependence of internal friction (the vibration decrement) of EK-181 steel after THT and HTMT are similar to those of its impact strength. Below the ductile–brittle transition temperature, it is characterized by a low level of dislocation internal friction. The temperature dependence curves of the steel elastic modulus increase monotonically with the decreasing temperature. In this case, the value of Young’s modulus is structure-sensitive. A modification of the microstructure of EK-181 steel as a result of HTMT causes its elastic modulus to increase, compared to that after THT, over the entire temperature range under study. The electron microscopic studies of the steel microstructure evolution near the fracture surface of the impact samples (in the region of dynamic crack propagation) in the temperature range from −196 to 100 °C reveal the traces of plastic deformation (increased dislocation density, fragmentation of the martensitic structure) at all of the temperatures under study, including those below the cold brittleness threshold of EK-181 steel.

The microstructure of low activation 12 % chromium ferritic-martensitic steel EK-181 (Fe-12Cr-2W-V-Ta-B) formed by multi-directional isothermal forging is studied. It is shown that the refinement of microstructural elements is followed by the formation of new ferrite grains and fragments of martensitic laths. The resulting finely dispersed particles of M23C6 carbides are homogeneously distributed. The mechanical properties under tensile test conditions and microhardness of steel EK-181 are studied. It is shown that multi-directional isothermal forging increases the strength properties by 30% at 20 degrees C. At 650 degrees C the strength properties of steel after multi-directional forging are comparable to those after traditional heat treatment. The microhardness of steel after thermomechanical treatment reaches 3.42 GPa, which is almost 20% higher than after traditional heat treatment. The microhardness values over the cross section of the workpiece do not change significantly. A high-temperature tempering at 720 degrees. for 1 hour after multi-directional isothermal forging leads to the formation of a submicrocrystalline ferrite structure, the growth of M23C6 carbides, reduction of dislocation density and decreases the strength properties.

The features of the microstructure and mechanical properties of the ferritic-martensitic steel EK-181 after warm isothermal forging in the ferritic region (tempforming) with the strain degree e ≈ 1.3 are studied. The formation of an inhomogeneous fragmented structure as a result of this processing is shown. The carbide subsystem does not change during processing. The mechanical properties of the steel have been studied under the conditions of tensile tests at 20 and 650°C accompanied by the microhardness measurements. It is shown that warm isothermal forging leads to some increase in the strength properties at room temperature. At an elevated temperature (650°C), the strength properties are comparable to those after traditional heat treatment (quenching and high-temperature tempering). The microhardness of steel after warm isothermal forging varied over the sample cross section from 2.7 GPa to 3.1 GPa. Tempering after multi-directional forging of the steel decreased its strength, but improved the plastic properties. The relationship between the microstructure and the mechanical properties of the EK-181 steel after isothermal forging in the ferritic region is discussed.